General
Boiler Information

1 pound of steam is about 1,200
BTUs of INPUT fuel, and about 1,000 BTUs at the point of use, depending
on the pressure of the steam

Low pressure steam is considered
to be up to 15 psi; high is generally 100 psi and higher.

Superheat is a term that refers
to higher temperature steam, as a result of a second special steam heat
exchanger in the boiler that allows steam pressure to increase, thereby
taking on more BTUs (in excess of 500 psi is typical of superheat)

Smaller boilers are generally
rated in horse power; larger are generally rated in thousands of pounds
of steam (500 hp and under will typically be rated in hp)

Typical boiler efficiency will
be in the 75 - 85% range; new highest efficiency boilers may be near 90%;
newer quick heat up types of boilers with copper heat exchangers are much
more efficient, especially at startup and part load than older, heavy
mass cast iron boilers

Common Boiler Types: (Click
on table for larger view)

Fire
Tube Boilers

The firetube derives its name
from the orientation by which heat is transferred to the fluid medium.
The boiler body itself is a pressure vessel containing the fluid which
is to be heated or physically altered. Within the vessel - and therefore
within the fluid - are tubes which waste gases pass through. Heat is transferred
from within the tube to the surrounding fluid; thus the name.

Advantages of this design are
not often realized intuitively when compared to the watertube. The volume
of fluid in the unit is roughly half the physical volume of the entire
boiler, thus control of water levels and chemistry is much more forgiving
and in need of less precision than for the typical watertube. Another
advantage is found in the ease with which a unit is manufactured and serviced.
The heat transfer tubes of the watertube boiler vary across the circumferential
span of the drums: there may be eleven or more differing tube types to
assemble in a single watertube unit. In the firetube, each heat transfer
tube is identical to its counterparts. Manufacture and service are readily
accomplished with a stock of a single tube type. It is this simplicity
which contributes to the profound difference in life-cycle and capital
cost of the firetube.

Disadvantages of the firetube
are attributable only to ability. The limits of design pressure, supplemental
firing rates, and capacity are realized more acutely with the firetube
than with the watertube. Design pressures above 400 psig begin to move
firetubes outside the realm of cost-effective design as do capacities
above 80,000 lb/hr and supplemental firing temperatures above 1,600 deg
F. While there are project-specific particulars which may or may not affect
the overall project feasibility of the firetube design, the watertube
often becomes the more reasonable option as operating parameters become
more adverse.

Water
Tube Boilers

The watertube is the most prevalent
heat recovery device in industrial applications and derives its name from
the orientation by which heat is transferred to the fluid medium. Flue
gas passes through the boiler transferring heat into tubes placed within
the gas path. Interior to the boiler tubes is the fluid medium to be heated
or physically altered; thus the name "watertube".

Steam
Information

Steam is an invisible gas that's
generated by heating water to a temperature that brings it to the boiling
point. When this happens, water changes its physical state and vaporizes,
turning from a liquid into a gas.

Conversely, when heat energy
is removed from steam, it loses its ability to retain a gaseous state
and condenses back into a liquid. We refer to the resulting liquid as
condensate. The temperature at which condensation takes place is known
as the dew point.

When water is heated at atmospheric
pressure, its temperature rises until it reaches 212°F (100°C), the highest
temperature at which water can exist at this pressure. Additional heat
does not raise the temperature, but converts the water to steam.

One pound of water takes 1
BTU per Degree of Temperature rise up to 212°F; to form steam, an additional
970 BTUs is required for the "Latent Heat of Vaporization".
Therefore, steam has (970 + (212 - Condensate Temperature)) BTUs
per pound. EXAMPLE: If the condensate temperature is 160°F,
(970 + (212 - 160)) = 1,022 BTUs per pound. This clearly shows why
steam has more energy content than hot water.

NOTE: These are BTUs
delivered to the water; efficiency must also be factored in to determine
INPUT BTU requirements.

Superheat

Superheat refers to the process
of increasing the temperature of steam above about 400°F and 100 psi.
This feature is most common in very large power plant boilers of
watertube construction. An additional heat exchanger capable of
the high temperatures and pressures is required.

At least one company promotes
a "direct fired" superheater, that could have some advantages
for smaller sized boilers that need higher temperatures and pressures,
but do not want to use a Thermal Fluid system.

According to their web site,
the Cannon Superheater addresses two distinct markets, new boilers and
retrofit installations. First, the addition of a Cannon Superheater allows
the installation of a boiler best sized for an installation. An
expensive watertube with an integral superheater could be put aside in
favor of a smaller firetube with the Cannon unit. The Cannon superheater
excels in the 25 HP to 1,000 HP range. Thermal oil systems, another way
to achieve higher temperatures, are now less attractive. Thermal oil systems
are expensive, not very flexible, and introduce environmental concerns.
Simple and inexpensive steam plants with a superheater can achieve the
same operating temperatures as the thermal oil system without their problems.
For more information, see www.cannonboilerworks.com,
Cannon Boiler's home page.

Boiler
Stack Economizer

Flue
gases from large boilers are typically 450 - 650°F. Stack Economizers
recover some of this heat for pre-heating water. The water is most
often used for boiler make-up water or some other need that coincides
with boiler operation. Stack Economizers should be considered as
an efficiency measure when large amounts of make-up water are used (ie:
not all condensate is returned to the boiler or large amounts of
live steam are used in the process so there is no condensate to return.)

The savings potential is based
on the existing stack temperature, the volume of make-up water needed,
and the hours of operation. Economizers are available in a wide
range of sizes, from small coil-like units to very large waste heat recovery
boilers.

Boiler
Water Treatment

Origin
of the Problem

The most common source of corrosion
in boiler systems is dissolved gas: oxygen, carbon dioxide and ammonia.
Of these, oxygen is the most aggressive. The importance of eliminating
oxygen as a source of pitting and iron deposition cannot be over-emphasized.
Even small concentrations of this gas can cause serious corrosion problems.

Makeup water introduces appreciable
amounts of oxygen into the system. Oxygen can also enter the feed water
system from the condensate return system. Possible return line sources
are direct air-leakage on the suction side of pumps, systems under vacuum,
the breathing action of closed condensate receiving tanks, open condensate
receiving tanks and leakage of nondeaerated water used for condensate
pump seal and/or quench water. With all of these sources, good housekeeping
is an essential part of the preventive program.

One of the most serious aspects
of oxygen corrosion is that it occurs as pitting. This type of corrosion
can produce failures even though only a relatively small amount of metal
has been lost and the overall corrosion rate is relatively low. The degree
of oxygen attack depends on the concentration of dissolved oxygen, the
pH and the temperature of the water.

The influence of temperature
on the corrosivity of dissolved oxygen is particularly important in closed
heaters and economizers where the water temperature increases rapidly.
Elevated temperature in itself does not cause corrosion. Small concentrations
of oxygen at elevated temperatures do cause severe problems. This temperature
rise provides the driving force that accelerates the reaction so that
even small quantities of dissolved oxygen can cause serious corrosion.

The Corrosion
Process

Localized attack on metal can
result in a forced shutdown. The prevention of a forced shutdown is the
true aim of corrosion control.

Because boiler systems are
constructed primarily of carbon steel and the heat transfer medium is
water, the potential for corrosion is high. Iron is carried into the boiler
in various forms of chemical composition and physical state. Most of the
iron found in the boiler enters as iron oxide or hydroxide. Any soluble
iron in the feed water is converted to the insoluble hydroxide when exposed
to the high alkalinity and temperature in the boiler.

These iron compounds are divided
roughly into two types, red iron oxide (Fe2O3) and black magnetic oxide
(Fe3O4). The red oxide (hematite) is formed under oxidizing conditions
that exist, for example, in the condensate system or in a boiler that
is out of service. The black oxides (magnetite) are formed under reducing
conditions that typically exist in an operating boiler.

External
Treatment

External treatment, as the
term is applied to water prepared for use as boiler feed water, usually
refers to the chemical and mechanical treatment of the water source. The
goal is to improve the quality of this source prior to its use as boiler
feed water, external to the operating boiler itself. Such external treatment
normally includes:

Any or all of
these approaches can be used in feed water or boiler water preparation.

Internal Treatment.

Even after the best and most
appropriate external treatment of the water source, boiler feed water
(including return condensate) still contains impurities that could adversely
affect boiler operation. Internal boiler water treatment is then applied
to minimize the potential problems and to avoid any catastrophic failure,
regardless of external treatment malfunction.

Feed Water
Preparation

The basic assumption with regard
to the quality of feed water is that calcium and magnesium hardness, migratory
iron, migratory copper, colloidal silica and other contaminants have been
reduced to a minimum, consistent with boiler design and operation parameters.

Once feed water quality has
been optimized with regard to soluble and particulate contaminants, the
next problem is corrosive gases. Dissolved oxygen and dissolved carbon
dioxide are among the principal causes of corrosion in the boiler and
pre-boiler systems. The deposition of these metallic oxides in the boiler
is frequently more troublesome than the actual damage caused by the corrosion.
Deposition is not only harmful in itself, but it offers an opening for
further corrosion mechanisms as well.

Contaminant products in the
feed water cycle up and concentrate in the boiler. As a result, deposition
takes place on internal surfaces, particularly in high heat transfer areas,
where it can be least tolerated. Metallic deposits act as insulators,
which can cause local overheating and failure. Deposits can also restrict
boiler water circulation. Reduced circulation can contribute to overheating,
film boiling and accelerated deposition.

The best way to start to control
pre-boiler corrosion and ultimate deposition in the boiler is to eliminate
the contaminants from the feed water. Consequently, this section deals
principally with the removal of oxygen, the impact of trace amounts of
contaminants remaining in the feed water, and heat exchange impact.

Feed water is defined
as follows:

Feed water (FW) = Makeup water
(MW) + Return condensate (RC)

The above equation is a mass
balance (pounds or kilograms).

Deaeration
(Mechanical and Chemical)

Mechanical and chemical deaeration
is an integral part of modern boiler water protection and control. Deaeration,
coupled with other aspects of external treatment, provides the best and
highest quality feed water for boiler use.

Simply speaking, the purposes
of deaeration are:

1. To remove
oxygen, carbon dioxide and other noncondensable gases from feed water2. To heat the incoming
makeup water and return condensate to an optimum temp3. Minimizing solubility
of the undesirable gases4. Providing the highest
temperature water for injection to the boiler

Deaerators

Mechanical deaeration is the
first step in eliminating oxygen and other corrosive gases from the feed
water. Free carbon dioxide is also removed by deaeration, while combined
carbon dioxide is released with the steam in the boiler and subsequently
dissolves in the condensate. This can cause additional corrosion problems.

Because dissolved oxygen is
a constant threat to boiler tube integrity, our discussion on the deaerator
will be aimed at reducing the oxygen content of the feed water. The two
major types of deaerators are the tray
type and the spray type.
In both cases, the major portion of gas removal is accomplished by spraying
cold makeup water into a steam environment.

Tray Type
Deaerating Heaters

Tray-type deaerating heaters
release dissolved gases in the incoming water by reducing it to a fine
spray as it cascades over several rows of trays. The steam that makes
intimate contact with the water droplets then scrubs the dissolved gases
by its counter-current flow. The steam heats the water to within 3-5 º
F of the steam saturation temperature and it should remove all but the
very last traces of oxygen. The deaerated water then falls to the storage
space below, where a steam blanket protects it from recontamination.

Nozzles and trays should be
inspected regularly to insure that they are free of deposits and are in
their proper position.

Spray-Type
Deaerating Heaters

Spray-type deaerating heaters
work on the same general philosophy as the tray-type, but differ in their
operation. Spring-loaded nozzles located in the top of the unit spray
the water into a steam atmosphere that heats it. Simply stated, the steam
heats the water, and at the elevated temperature the solubility of oxygen
is extremely low and most of the dissolved gases are removed from the
system by venting. The spray will reduce the dissolved oxygen content
to 20-50 ppb, while the scrubber or trays further reduce the oxygen content
to approximately 7 ppb or less.

During normal operation, the
vent valve must be open to maintain a continuous plume of vented vapors
and steam at least 18 inches long. If this valve is throttled too much,
air and nonconclensable gases will accumulate in the deaerator. This is
known as air blanketing and can be remedied by increasing the vent rate.

For optimum oxygen removal,
the water in the storage section must be heated to within 5 º F of the
temperature of the steam at saturation conditions. From inlet to outlet,
the water is deaerated in less than 10 seconds.

Deaerators
and Economizers

Where economizers are installed,
good deaerating heater operation is essential. Because oxygen pitting
is the most common cause of economizer tube failure, this vital part of
the boiler must be protected with an oxygen scavenger, usually catalyzed
sodium sulfite. In order to insure complete corrosion protection of the
economizer, it is common practice to maintain a sulfite residual of 5-10
ppm in the feed water and, if necessary, feed sufficient caustic soda
or neutralizing amine to increase the feed water pH to between 8.0 and
9.0.

Below 900 psi excess sulfite
(up to 200 ppm) in the boiler will not be harmful. To maintain blowdown
rates, the conductivity can then be raised to compensate for the extra
solids due to the presence of the higher level of sulfite in the boiler
water. This added consideration (in protecting the economizer) is aimed
at preventing a pitting failure. Make the application of an oxygen scavenger,
such as catalyzed sulfite, a standard recommendation in all of your boiler
treatment programs.

Treatment

The foregoing discussion shows
the importance of proper deaeration of boiler feed water in order to prevent
oxygen corrosion. Complete oxygen removal cannot be attained by mechanical
deaeration alone. Equipment manufacturers state that a properly operated
deaerating heater can mechanically reduce the dissolved oxygen concentrations
in the feed water to 0.005 cc per liter (7 ppb) and 0 free carbon dioxide.
Typically, plant oxygen levels vary from 3 to 50 ppb. Traces of dissolved
oxygen remaining in the feed water can then be chemically removed with
the oxygen scavenger.

Blowdown
Control

The main purpose of blowdown
is to maintain the solids content of the boiler water within prescribed
limits. This would be under normal steaming conditions. However, in the
event contamination is introduced in the boiler, high continuous and manual
blowdown rates are used to reduce the contamination as quickly as possible.

Because each boiler and plant
operation is different, maximum levels should be determined on an individual
basis.

Bottom Blowdown

By definition, bottom blowdown
is intermittent and designed to remove sludge from the areas of the boiler
where it settles. The frequency of bottom blowdown is a function of experience
and plant operation. Bottom blowdown can be accomplished manually or electronically
using automatic blowdown controllers.

Continuous
Blowdown

Frequently used in conjunction
with manual blowdown, continuous blowdown constantly removes concentrated
water from the boiler. By design, it is in the area of highest boiler
water concentration. This point is determined by the design of the boiler
and is generally the area of greatest steam release.

Continuous blowdown allows
for excellent control over boiler water solids. In addition, it can remove
significant levels of suspended solids. Another advantage is that the
continuous blowdown can be passed through heat recovery equipment.

Blowdown
Control Summary

Proper boiler blowdown control
in conjunction with proper internal boiler water treatment will provide
the desired results for a boiler water program. Many modern devices can
automate boiler blowdown, thereby increasing the overall efficiency of
the unit.

ION EXCHANGE SYSTEMS

Ion exchange systems range
from light commercial water softeners and filters to specially designed
industrial equipment. Also known as deionizations (DI) systems. These
systems are considered high-end where the highest quality of water treatment
is needed, such as with steam turbines.

RO SYSTEMS

Reverse Osmosis (RO) systems
are available for tap water, brackish water or seawater. These systems
are considered high-end where the highest quality of water reatment is
needed, such as with steam turbines.